Magnetic bearings experience less loss than traditional hydrodynamically lubricated bearings. Being dry, they can be used in a clean atmosphere. They are also particularly practical for use in a vacuum state. For these reasons, magnetic bearings have come to be widely used to support the rotating parts of high-speed rotor units driven directly by motors, such as the rotors in turbo molecular pumps.
A high-speed rotor unit supported by magnetic bearings, such as a turbo molecular pump, would typically have the configuration shown in FIG. 9. Pump unit A and power supply unit B are electrically connected via cables 40. On side A are the pump itself; motor 29, which is directly connected to the pump; magnetic bearings 9, which maintain the position of the rotor of the motor 29 and the rotating portion of the pump connected directly to that rotor; and position sensors 24, which detect the gaps between the bearings and the rotor.
Power supply unit B comprises motor drive circuit 34 (a power supply circuit which possibly includes an inverter circuit) for driving motor 29, which is directly connected to the rotor; magnetic bearing control circuit 32 and magnetic bearing drive circuit 33, which receive the signals from the aforementioned position sensors 24 and control the drive voltages to magnetic bearings 9 (electromagnets) so as to keep the rotor centered; and magnetic bearing power supply 31, which supplies power to drive the magnetic bearings. Units A and B are connected by cables 40A through 40C.
Each of the control and drive circuits, 32 and 33, in power supply unit B configured as described above must be adjusted or set according to the type of pump unit used.
Magnetic bearing Control circuit 32 controls the magnetic bearings to keep the rotor centered in one embodiment of this invention which will be discussed shortly. This magnetic bearing control circuit comprises, as shown in FIG. 4 (A), a PID control circuit 62 equipped with a comparator element, an integrator element and a differentiator element; a phase compensator circuit 63, which stabilizes the vibrations with a plurality of dimensions resulting from the configuration of the rotor; filter circuit 61 (a notch filter, a low pass notch filter, a low pass filter or a phase filter); and a bias circuit 65, which applies bias current in order to assure that magnetic bearings 9 can rigidly bear the static load imposed by the rotor. Each of these circuits must be adjusted by setting different constants depending on the type of rotor unit being driven.
Also in motor drive circuit 34, if the inverter is to be operated such that V/f remains constant, not only the constants needed to control rotation by maintaining the rated voltage and frequency, but also various additional constants must be set to protect the rotor unit from overvoltage, overcurrent, overload and excessively high frequency inputs.
For this purpose, limit element circuit cards 32A to 34A are built into control circuit 32 and drive circuits 33 and 34 in power supply unit B for each type of rotor unit.
In prior devices, then, the pump unit A for a given type of rotor unit must be matched with a particular power supply unit B. The proper units A and B must be selected during assembly, and a power supply unit B and pump unit A selected for one type of rotor unit cannot be used for another type. Thus a different power supply unit must be selected for the pump unit used for each type of rotor unit. This severely compromises productivity. Such variation also poses a problem in terms of aftermarket administration and troubleshooting. If a number of different types of turbo molecular pumps are used, it becomes difficult to design the layout for the rotor units and the plant in which they are used. The installation will occupy a great deal of space, and maintenance will be troublesome.
To address these shortcomings, the present applicant proposed in Japanese patent application 8-317876 that the circuits in the power supply unit B be designed so as to limit the number of rotor unit types for which unit B would be applicable. According to the proposed design, which can be seen in FIG. 8. element circuit cards 32A through 34A for each type of rotor unit are provided in pump unit A for bearing control circuit 32 and bearing and motor drive circuits 33 and 34. Although this design allows power supply unit B to be used generally, it requires that each circuit such as bearing control circuit 32 and motor drive circuit 34, be built into the unit for each type of rotor unit, which limits the interchangeability of pump unit A,. This raises the production cost and increases the size of the hardware which constitutes pump unit A, which prevents the entire device from being streamlined. Another problem is that turbo molecular pumps are sometimes placed in the vicinity of equipment used in the production of semiconductors, particularly equipment where reactive substances are present such as vacuum containers (or chambers). The equipment or countermeasures required to maintain reliability when such circuit boards are installed in such a location are very costly.